If my mother claimed me as a dependent last year and received stimulus for me, why does it say that I qualify for the stimulus?

It says that because you do qualify for the stimulus based on your 2020 tax return, provided that no one can claim you as a dependent for 2020. There are several situations like this where two people can get stimulus money for the same person. That's the way the stimulus payments work. The IRS knows it happens, and it's okay.

Note that the requirement for you to qualify for the stimulus for 2020 is that no one can claim you as a dependent. If your mother, or anyone else, could claim you, you are not entitled to a stimulus payment. It doesn't matter whether they actually do claim you.

TurboTax starts out assuming that you qualified for the stimulus and that you did not receive it.  If you are not able to be claimed as a dependent for 2020, then you may qualify for the second stimulus.  You did not qualify for the first, because you were your mother's dependent.

Because TurboTax does not know that your mother claimed you last year, it is saying that you qualify for the payment.  But you know that is not correct.  The best way to resolve this is to put that you received the 2019 stimulus payment and the amount that TurboTax is trying to add to your refund.  That will correct the issue and you will not get a stimulus payment added to your return that you would end up having to pay back.